Heretofore, the development of communications and radar using short-pulse signals has been carried out as one UWB (Ultra Wide Band) technology. Methods of making a short-pulse signal having only a desired frequency band component are to extract only a specific frequency component by performing frequency band limiting of a pulse signal by means of a filter, or to obtain short pulses by intermittent operation of an oscillator by inputting a pulsed control signal. There is also a method whereby intermittent circuit operation is performed by inputting a pulsed control signal to an amplifier or frequency multiplier, and a short pulse is generated that corresponds to an interval in which the voltage level of the control signal is high.
An oscillator is composed of an active element such as a transistor, a negative resistor comprising a tunnel diode or operational amplifier, and a resonant circuit. A conventional intermittent operation circuit using a tunnel diode oscillator will be described using FIG. 1. In intermittent operation circuit 10, a control signal output from a control signal generation circuit 11 is input to a negative resistor 13 via an inductor 12. Negative resistor 13 has the kind of voltage-current characteristic indicated by reference code 14 in FIG. 1, with oscillation performed when the voltage of the input control signal is located in area 1, and oscillation stopping when the voltage of the input control signal is located in area 2 or area 3 (see Patent Document 1).
However, with the above-described conventional circuit configuration, when a pulsed control signal is input to the circuit, the control signal waveform is rounded due to floating capacitance or parasitic capacitance present in the circuit. Consequently, there is a period of time until a steady state is reached after the control signal is input (rise time), and a period of time until oscillation stops after the control signal is stopped (fall time), as a result of which the rise and fall of the output waveform is rounded. This problem is not limited to a case in which a tunnel diode is used as a negative resistor, but also occurs in a similar way in an oscillator that uses a transistor. Furthermore, this problem is not limited to an oscillator, and the same kind of problem also occurs in intermittent operation of an amplifier or frequency multiplier.
Heretofore, one known method of solving this problem, as shown in FIG. 2, has been to use a waveform 21 in which overshoot 22 and undershoot 23 are caused in the rise and fall areas as a control signal. By means of overshoot 22 and undershoot 23, waveform rounding due to input capacitance (inter-terminal capacitance or floating capacitance) present in an oscillator, amplifier, or frequency multiplier can be canceled, enabling the rise time and fall time of the output waveform to be effectively shortened.
However, with this method, the output waveform does not rise at high speed at or above the gradient of the control signal, and ideal waveform shaping is difficult. The use of a speed-up capacitor to generate overshoot and undershoot is known as a means of performing waveform shaping easily, but since a self-resonant frequency is present in a capacitor, there are limits on the frequency components of generated overshoot and undershoot, and consequently the control signal does not have a sufficiently steep waveform. Therefore, although the waveform has a smooth rise and fall, as shown by waveform 31 in FIG. 3, and there is an input capacitance charging effect, a nanosecond-order rise characteristic is not obtained. In the case of an intermittent operation circuit, in particular, it is difficult to generate short pulses that can be used in UWB because of delay in the feedback circuit of the circuit.
A method of solving this problem, as shown in FIG. 4, is to input a short-time-width startup boosting signal having the same frequency components as an oscillator 43 that includes a crystal resonator 42, generated by a standard signal generation apparatus 41, to oscillator 43 based on a timing waveform from a timing generation circuit 44 (see Patent Document 2). This enables the growth of oscillation to be boosted at the time of an output waveform rise, and is therefore effective in achieving high-speed oscillation startup.
Patent Document 1: National Publication of International Patent Application No. 2003-513501
Patent Document 2: Unexamined Japanese Patent Publication No. HEI 3-231504